It has already been known for a long time that the photometric parameters of gas discharge lamps can be improved substantially with the aid of a pulsed supply current. These methods have gained particular attention in sodium vapor high-pressure lamps, in which the color temperature can be raised from approximately 2000 to above 3000K by strobing. At the same time, an improvement in color reproduction is possible. German Patents DE-PS 26 57 824 and DE-PS 28 25 532 refer to these subjects.
When sodium vapor discharge lamps are operated with a plurality of metal vapors, bipolar impulses must be used, in order to prevent separation phenomena.
The nature of the strobe pulses, their shape, the pulse width, the duty factor, the rise and the height have a decisive influence on the attainable photometric parameters. In most versions, work is preferably done with relatively wide pulses (approximately 100 to 200 .mu.s) and duty factors of approximately 0.2, with repetition rates below 500 Hz, as described in German Patent DE-PS 36 36 901.
At frequencies above 500 Hz, the discharge system has a tendency to develop resonant phenomena, which are expressed in the form of severe arc instabilities and losses in light yields (German Patent DE-PS 31 22 183). It is also known for the pulse geometry and pulse shaping to be formed in such a way, when electronic ballasts are used, that the pulses have a rapid transition from the cooling phase to the state of a predetermined pulse output, a prolonged operative time at that level, and a rapid return to the state of the cooling phase. For pulse shaping, a multiple-member delay line is proposed, comprising capacitive and inductive components. Experiments have shown that the optimal pulse width for sodium vapor discharge lamps is between 100 and 200 .mu.s, so that if delay lines are used for pulse shaping, a suitable electronic ballast would be very expensive in terms of components and would be large in volume.
In principle it can be stated that generating relatively wide pulses of appropriate height makes major demands of ballast technology, because large quantities of energy must be stored and switched.
Electrical circuits for the pulsed operation of high-pressure gas-discharge lamps are known for instance from German Patent Disclosure DE-OS 36 41 070, German Patent DE-PS 35 40 985, and German Patent DE-PS 26 57 824.
A substantial disadvantage of these electrical circuits is the major technological effort and expense entailed in making them, and the great losses that must sometimes be expected in the current-limiting components. Moreover, a constant load can be supplied only by expanding the circuit, which is expensive.